Electrochemical synthesis of poly(3-thiophene acetic acid) nanowires with water-soluble macromolecule templates

Abstract

Poly(3-thiophene acetic acid) (PTAA) nanowires are synthesized in a high yield by electrochemical polymerization, with diameters of about 200 nm and lengths of tens of micrometres. In the reaction systems, the water soluble macromolecules hydroxyethylcellulose (HEC), chitosan (Cs) and polyacrylamide (PAM) are added respectively as templates through a dip-coating process, where the macromolecules are spread on an indium tin oxide glass which is used as the working electrode. However, when polyethylene glycol (PEG) is used, only a PTAA film can be obtained. Therefore the interactions between template macromolecules and TAA monomers play a key role in the formation of PTAA nanowires and the dip-coating process presents an effective way to produce 1D conductive polymer materials with controllable shape. The optical, electrochemical and surface properties are also studied respectively to compare the performances of PTAA film and nanowires, where the latter indicates good electrochemical responsiveness and evidently increased hydrophobicity.

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1. Introduction

One-dimensional (1D) conducting polymer (CPs) materials, such as nanofibers, nanotubes, and nanowires have attracted a great deal of attention in recent years.^1–8 This is mainly because of their unique morphologies, properties and the applications in sensors,^1,2 supercapacitors,^3,4 and actuators^5,6 et al. Therefore, considerable efforts have concentrated on the construction of ordered 1D nanostructures of CPs.

Up to now, a number of techniques have been developed for nanostructures of CPs, such as template-assisted polymerization,^9–17 lithography, interfacial polymerization,¹⁸ electrospinning^19,20 and seeding polymerization.^21–25 Recently, the “soft templates” method is well-known in the preparation of organic nanostructures, which usually utilizes molecular assemblies as templates, such as micelles, microemulsions or liquid crystalline phases, to guide the growth of the structures of conducting polymers.^16–19,26 For instance, Wu et al.¹⁶ reported recently a mechanistic study on the nucleation of polyaniline nanotubes by in situ solution-state ¹H NMR experiments. Their results showed that loosely packed anilinium composed micelles is one of key factors for the generation of tubular structures. Zhang and Manohar et al.^21–25 described an extremely simple “nanofiber seeding” method to synthesize bulk quantities of nanofibers of the conducting polymers (polyaniline, polyppyrole, polythiophene), where the seed nanofibers acting as the template initiated fibrillar polymer growth and resulted in bulk polymer nanofibers. Hamaguchi et al. obtained poly(N-methylpyrrole) tubes with nanoscaled inner holes and walls by using a self-assembly process,¹⁹ where the self-assembled local structures in the solvent ionic liquid are likely to serve as templates. Generally, when these templates are used, the monomers can be grown along their structures during polymerization, resulting in the formation of the 1D CPs nanostructures. Because the soft template are usually made of surfactants or organic molecules, so the method has notable advantages because it avoids the complicated template-removal step.

In recent years, polythiophene and its derivatives show attractive performance in sensors,^27,28 solar cells,^29,30 transistors³¹ and photovoltaics³² et al. As one of important members, poly(3-thiophene acetic acid) (PTAA) has been studied about its synthesis. PTAA is an intrinsically conducting polymer with various interesting properties such as high conductivity, good thermal and environmental stability, and biocompatibility. Nanostructured PTAA is useful for fabricating microdevices including reactors, actuators and sensors.^40–43 Because 3-thiophene-acetic acid (TAA) is insoluble in conventional aqueous media and also its oxidation potential is higher than that of water decomposition, PTAA is generally electrosynthesized in organic media such as boron trifluoride diethyl etherate (BFEE) and trifluoroacetic (TFA),^33–36 which furnishes a conducting medium. The strong electrophilic property of this Lewis acid can strongly catalyze the deprotonation of aromatic compounds at electrodes and decrease the oxidation potential of aromatic compounds dramatically.³³ Electrodeposition may be regarded as a reliable method in order to control the size and morphology of the electrodeposits and has been applied recently to produce PTAA films.^37–39 However, compared to polypyrrole and polyaniline, reports about the preparation of PTAA with nanostructures are few and the PTAA nanowires is reported for the first time. Moreover, the synthesis of straight and orderly PTAA nano/micro structures using templates mentioned above is difficult, because that the structures of assembly as template are also “soft” and easy to be changed by conditions such as composition or temperature. By comparison, the molecule template is simple and easy to be controlled. In our previous work,⁴⁴ a dip-coating process was successfully used to spread water soluble macromolecule HEC on the surface of working electrode and further to prepare template for the electrochemical polymerization of highly ordered and straight PTAA microwires. Here, the simple dip-coating approach for preparing water-soluble polymers template is utilized to control the morphologies of PTAA and consequently the properties of products. The dip-coating process^45–47 involves immersing a ITO into template solution for some time thereby ensuring that the ITO is completely wetted, and then withdrawing the ITO from the solution bath. The liquid film formation is achieved by two main mechanisms, i.e. gravity and evaporation of solvent. The quality of the template are controlled by the solution concentration and pulling speed. Generally, high template concentration and slow pulling speed will get a high quality of template. One of the important aims of this work is to study if the method is generally effective in controlling the microstructure of different CPs. Water-soluble macromolecules HEC, Cs, and PAM are chosen to prepare the template respectively, because they possess chainshaped structure with many hydroxy groups (–OH) and amino groups (–NH₂) which can combine TAA molecules through hydrogen bonds.

In this work, the obtained PTAA nanowires is very stable and show excellent electrochemical properties, which makes it an ideal substrate for electrochemical detection and great promise for biosensing.⁴⁸

2. Experimental details

Boron trifluoride diethyl etherate (BFEE) is a product of Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and chemically pure. Trifluoroacetic acid is purchased from Aladdin and used without further treatment. Thiophene-3-acetic acid (98.0%) is purchased from TCI. Chitosan (CS) and polyacrylamide (PAM) are both purchased from Sinopharm Chemical Reagent. Hydroxyethyl cellulose (HEC) is purchased from Aladdin. Ultrapure water is used throughout this study.

The basic strategy for electropolymerization of PTAA nanowires in the presence of water-soluble macromolecule templates are based on the steps reported in our previous work.⁴⁹ The first step is the preparation of template. The 0.02 M phosphate buffer solution (pH = 6.86) containing 0.01 wt% water-soluble macromolecules is prepared to stand at room temperature in a sealed glass container for 24 h as the coating solution for dip-coating process. Then an ITO glass perpendicular to the surface of the coating solution is immersed into it for 2 minutes and then pulled out of the solution carefully with a low pulling speed. Subsequently, the as prepared ITO glass is used as the working electrode and immersed in BFEE (75%) mixed with 25% (by volume) TFA and containing 0.05 M thiophene-3-acetic acid. Then the thiophene-3-acetic acid monomers are polymerized at a potential of 0.7 V for 300 s. All samples are washed with water in order to remove electrolyte and thiophene-3-acetic acid monomer.

The morphologies of PTAA products are measured with scanning electron microscopy (SEM, LEO1530, Germany). The reflectance Fourier-transform infrared spectroscopy (FTIR, Avatar 360, Nicolet, America) is used to characterize the composition of PTAA samples.

The electrochemical measurement including cyclic voltammetry (CV) is carried out by using LK2005A electrochemical workstation. The CV tests are conducted in a solution containing 5 mM K₃[Fe(CN)₆] and 0.2 M KCl (scan rate = 50 mV s⁻¹). Contact angle measurements are performed at a room temperature of 296 K using an OCA40 type of wetting angle measuring instrument made in German.

3. Results and discussion

3.1 Formation of PTAA nanowires

Electrochemical polymerization of 0.5 M thiophene-3-acetic acid in BFEE (75%) and TFA (25%) at different applied potentials led to formation of a blue-black film on the surface of the working electrode. When the ITO glass is put in the air, the film with the blue-black color gradually become yellow.

Fig. 1 shows typical SEM images of PTAA microstructures obtained by polymerization of 0.05 M thiophene-3-acetic acid in BFEE (75%) and TFA (25%) at a potential of 0.7 V for 300 s. When no HEC molecules are added into the reaction system, there are no wire-like structures on the PTAA film surface (Fig. 1a). It is clear from Fig. 1b that the nanowires with diameters of about 250 nm and lengths of micrometers are formed in a high yield. And also it is seemed that the wires are linked together to form a net structure. The SEM image (inset in Fig. 1b) is the low magnification PTAA nanowires.

Fig. 1 SEM images of normal PTAA (a) and PTAA nanowires synthesized through the special HEC template (b). Inset is the low magnification of (b).

When the reaction time is changed to 60 s with other conditions unchanged, PTAA nanowires can also be produced. As shown in Fig. 2a, the sample growth with the shortest time, 1 min, small diameters of nanowires growth are observed at the early stages of nanowires formation. It is evident from this image that the diameter of nanowires growth on ITO is 100 nm. In Fig. 1b a much longer deposition time of 5 min is used. The growth after this period of times, the nanowires diameter increase from 100 nm to 250 nm and indicates that polymerization has predominantly occurred at this stage. The image also shows that during this period, very large growth has occurred in the diameter of nanowires. In another experiment (micrograph not shown) we confirm that changing the voltage has little impact on the morphology of PTAA nanowires.

Fig. 2 SEM images of PTAA nanowires deposited on ITO glass with growth times of (a) 1 min. Monomer concentration of (b) 0.05 M. Template molecule concentration of (c) 0.01% and (d) 0.1%. Inset is the low magnification of image (c).

Noticeably, our results show that the morphology of the PTAA nanowires vary slightly as we change the monomer concentration from 0.1 M (Fig. 1b) to 0.05 M (Fig. 2b), other conditions are the same. Obviously the wire-diameter of the PTAA nanowires is slightly changed; it became smaller as the TAA concentration decreased from 0.1 M to 0.05 M. This indicates that the wire-diameter is not only dependent on the growth time but also the monomer concentration.

To investigate the effect of the concentration of template molecule on the growth of the nanowires, we performed a series of experiments under identical electrochemical conditions but with different concentration of template molecule. Fig. 2c and d shows SEM images of PTAA nanowires in low (0.01%) and high (0.1%) concentration of template molecule, respectively. Owing to the increased concentration of template molecules, the Fig. 2c (inset is the low magnification of Fig. 2c) shows a completely different morphology from Fig. 2d. The SEM image (Fig. 2d) clearly reveals that highly ordered network structure of PTAA nanowires, which structure has highly porous structure and high surface area. These characteristics displayed remarkably improved electrochemical performances. Therefore, the amount of PTAA nanowires depends on the concentration of template molecule.

Therefore, the HEC molecules play an important role as the template molecule in the formation of PTAA nanowires. Because stronger hydrogen bondings can be formed between HEC and TAA monomers, this results in the arrangement of TAA along the HEC chains which have been spread orderly on ITO glass when the glass is pulled out of the coating solution during the dip-coating process. Consequently, PTAA nanowires can be produced. The role of HEC played in this reaction system is just like that played by the “seed template” in the “nanofiber seeding” method reported by Manohar and Zhang et al.^21–25

To further confirm the effect of water-soluble macromolecules as the template on the formation of PTAA nanowires, further experiments are performed. Firstly, other two template molecules containing PAM and Cs are chosen to synthesize nanowires. PAM and Cs molecules have chain shaped structures with many –OH groups and –NH₂ groups (see Scheme 1), these groups can interact through hydrogen bonding interactions with –OH groups of TAA monomers. As shown in Fig. 3a and b, the PTAA nanowires obtained from these reaction systems can be seen, although their morphologies are not completely the same. Secondly, in another experiment, the mPEG molecule is chosen as the template. The morphology of such produced PTAA is shown in Fig. 3c, and it can be found that there are no nanowires structures on the surface of PTAA film.

Scheme 1 Diagrams of water-soluble macromolecules structures (HEC, Cs, PAM and mPEG).

Fig. 3 SEM images of nanowires synthesized through different systems containing (a) PAM (inset is the low magnification of (a)), (b) Cs (inset is the higher magnification of (b)), and (c) mPEG as templates, respectively. Other conditions are the same with Fig. 1b.

These results further confirm the formation mechanism of PTAA nanowires mentioned above and reported in the previous work.⁵⁰ When the water-soluble macromolecules template (as the working electrode) is immersed into the solution containing TAA, monomers will be spontaneously adsorbed onto water-soluble polymer molecules through hydrogen bonds formed between TAA molecules and the –OH or –NH₂ groups of template molecules. Finally, the absorbed monomers are polymerized preferentially along the chains of template to form PTAA nanowires (Scheme 2). Therefore, the orderly arrangement of water-soluble macromolecules on the ITO glass surface plays the principal role in the formation of PTAA nanowires. Because mPEG molecules contains only one –OH group on the end of the molecule chain (see Scheme 1), therefore strong interactions can not be formed between mPEG molecules and TAA monomers, which leads to no wire-like products.

Scheme 2 Diagram of the synthetic process of PTAA nanowires and the hydrogen-bond network between thiophene-3-acetic and water-soluble macromolecules.

3.2 FTIR spectra characterization

Fig. 4 and 5 show the FTIR spectra of the prepared PTAA film and PTAA nanowires. As shown in Fig. 4, the characteristic peak at 3442 cm⁻¹ is attributed to –OH group, the 2931 cm⁻¹ band can be assigned to the C–H stretching mode, the strong band at 1715 cm⁻¹ is attributed to the carbonyl group (C=O),^51,52 the peaks at 1407 cm⁻¹ is associated with the ring stretching of TAA, while the peak at 780 cm⁻¹ attributing the Cα–H stretching mode of the thiophene ring has no appear. These spectral results demonstrated that the PTAA were grown through the Cα–Cα coupling of the monomers. In comparison with the FTIR spectrum of normal PTAA film (Fig. 4), the spectra of nanowires obtained respectively from different systems (Fig. 5) containing HEC, Cs and PAM are the same with that of PTAA, which confirms the formation of wire-like PTAA.

Fig. 4 FTIR spectrum of PTAA and schematic diagram of polymerization.

Fig. 5 FTIR spectra of PTAA nanowires synthesized through systems containing HEC (a), Cs (b), and PAM (c) as template, respectively.

3.3 Optical properties

PTAA film and PTAA nanowires can partly dissolve in solvent dimethyl sulfoxide (DMSO). Under the 365 nm wavelength of ultraviolet light, the DMSO solution of 0.1 M TAA shows pale blue fluorescence, as can be seen in Fig. 6a. The DMSO solutions of partially soluble PTAA and PTAA nanowires show light green or yellow fluorescence (Fig. 6b–e), respectively. The inset in Fig. 7 indicates that the UV-vis absorption spectrum of TAA monomers presents two absorption peaks with the biggest absorption at 260 nm. Fig. 7a–d show that the absorption peak of PTAA and PTAA nanowires is at about 402 nm (Fig. 7). These results suggest that nanowires are made of PTAA and have a wide chain length distribution.

Fig. 6 Optical images of DMSO solution contains 0.1 M TAA (a) and partly dissolved PTAA film (b) and partly dissolved PTAA nanowires synthesized respectively through HEC (c), PAM (d) and Cs (e) systems.

Fig. 7 The UV-vis absorption spectra of different samples with PTAA nanowires synthesized respectively through Cs (a), PAM (b) and HEC (c) systems, partly dissolved PTAA film (d) and TAA (inset) (solvent: DMSO).

The TAA emission peak of fluorescence spectrum appears in a shorter wavelength range (inset in Fig. 8). The fluorescence curves of partially soluble PTAA and nanowires have more than two emission peaks at longer wavelengths, compared with the monomer's peak. This suggests that PTAA has an effective conjugated chain length and it can make stronger and single fluorescent. It can be noticed that the curves of PTAA nanowires obtained from Cs system and PTAA film are a little similar, while those of nanowires produced from HEC and PAM systems are also alike. These results are consistent with the morphologies presented in Fig. 2 and 3, where the nanowires synthesized from Cs system are connected together. Additionally, from Fig. 8a–d, slight red shifts can be seen of emission peaks from the curves, which can be ascribed to different elongation of the PTAA's delocalized π-electron chain sequence.⁵³

Fig. 8 The fluorescence spectra of TAA (inset) and partly dissolved PTAA film (a) and PTAA nanowires synthesized respectively through Cs (b), HEC (c), and PAM (d) systems (solvent: DMSO).

3.4 Electrochemical properties

As shown in Fig. 9, the CV of the PTAA film with nanowires (2 cm²) are carried out in a solution containing 5 mM K₃[Fe(CN)₆] and 0.2 M KCl at a scan rate of 50 mV s⁻¹ (at room temperature) shows a pair of strong and broad oxidation and reduction waves at ca. 0.25 V and 0.17 V. Comparing the four CV curves, it can be concluded that PTAA nanowires have a higher electrolyte capacitive charges, which is mainly due to the fact that the nanowires have a much larger surface area than a normal film. Especially, among these nanowires synthesized from different template systems, those produced by HEC template has the highest electrolyte capacitive charges.

Fig. 9 Cyclic voltammograms of PTAA nanowires synthesized respectively through Cs (a), HEC (b) and PAM (d) systems, and PTAA film (c).

3.5 Water drop contact angle (WDCA)

The increase in surface hydrophobicity has often been attributed to an increase in surface roughness.⁵⁴ The roughness of PTAA film surface can be increased by the formation of nanowires, which is reflected by the hydrophobicity of the film surface. Fig. 10 indicates optical images of water droplets on the surface of the PTAA films with nanowires. The water contact angle of PTAA film with nanowires from HEC systems is the biggest (95.5°) and that from Cs system is the smallest (76.0°) (water drops rapidly spreading on the surface of normal PTAA film). This is consistent with the morphology presented in Fig. 1–3, where the more nanowires in Fig. 2d increase the roughness of the PTAA surface, comparing to the PTAA film without wires.

Fig. 10 Photograph of water drop on PTAA nanowires deposited on the glass substrate synthesized respectively through HEC (a), Cs (b) and PAM (c) systems.

4. Conclusions

In summary, PTAA nanowires can be generated by electropolymerization of TAA through different water-soluble macromolecule template by a dip-coating process. The interactions between template molecules and monomers play the key role in the formation of PTAA nanowires. The obtained PTAA nanowires show increased hydrophobicity, evidently good electrochemical and optical properties, due to their orderly arranged wire-like microstructures and large surface areas, which makes it an ideal substrate for electrochemical detection and great promise for biosensing. The result suggests that the method can not only be used to prepare highly ordered PTAA nanowires, but also provide an effective way to produce 1D CPs with controllable shape and to understand the mechanism underlying the formation of their morphology. In addition, the technology developed by this work can be extended to synthesize other materials with similar microstructures by changing the monomers and templates.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (51003040), Shandong Provincial Natural Science Foundation, China (ZR2012BL11), and Shandong Provincial Science and Technology Development Plan Project, China (2013GGX10705).

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